Ship Pressure Relief Valve: Design and Operation

The machinery systems on board the ship include several safety features to ensure the ship’s safe and smooth operation, as well as the safety of its crew. A relief valve is an important safety device that is used in almost all machinery systems that deal with extremely high pressures.

These high pressures frequently exceed the predetermined limits, and the pressure that results is just above the maximum allowable working pressure (MAWP) or designed working pressure.

Onboard a ship, there are systems that use pneumatics or hydraulics, and sometimes even air or electronics. As a result, the construction and operation of the Pres Vac  valve should be such that even if the control system fails, the relief valve must lift to protect the system from over-pressurisation.

What Is the Function of a Pressure Relief Valve?

The Pres Vac valve operates through the action of a spring that is controlled by an operating pressure. The spring’s opening pressure can be adjusted using an adjusting screw located on top of the relief valve. Because the spring acts in the opposite direction of the pressure, the valve will not operate during normal operation of the machinery.

When the pressure acting on the valve seat rises above normal and equalises with the downward force of the spring, the relief valve will lift and release the excess pressure until equilibrium is reached.

The valve’s lifting pressure can range from 8-15 percent of the working pressure for an unfired system to the manufacturer’s recommendations. As soon as the system pressure returns to normal, the spring that was set to lift at a specific pressure will close the valve due to spring tension, and the machinery or system will resume normal operation.

When the system is continuously operating in an overpressure situation, valves are designed to have a full lift to release the excessive pressure. Safety valves and relief valves are both examples of these types of valves.

Furthermore, various safety codes and standards are written to control the design and application of the relief valve so that even if other safety systems fail, the relief valve will operate to prevent a disaster.

Construction of Relief Valve

Furthermore, various safety codes and standards are written to control the design and application of the relief valve so that even if other safety systems fail, the relief valve will operate to prevent a disaster.

The following sections of a relief valve are described:


The relief valve’s body is typically made of cast steel. It includes all of the components such as the valve spindle, valve, spring, and seat. It must be strong enough to withstand the high pressure generated when valves open to release excess pressure through the body.

Inlet and Outlet Connection

The relief valve inlet is connected to the machinery or system, and its outlet connection can be open to the atmosphere near the system only in the case of an air system, driven through a duct outside the engine room normally in the case of a steam system, or connected to the inlet or to some reservoir in the case of a hydraulic system.


The diaphragm acts as a seal between the inlet outlet connection and the valve body, preventing media from leaking through the valve body when the relief valve is activated.

Valve Seat

The seat must be soft enough not to damage the valve and durable enough to last for a longer period of time, otherwise the media for which the valve is used will leak. To combat pressure and corrosion, the seat is usually made of stainless steel coated with soft metal.


The valve is critical for the controlled operation of the relief valve, and its failure will result in media leakage from the machinery or system. Stainless steel is typically used.

Spindle/ Plunger

The valve is attached to the bottom of the spindle/plunger, which is also known as the valve stem, and the spring acts on top of it. The spring’s force is transmitted to the valve via the spindle. Stainless steel is used for the spindle.

Spring and Adjusting Nut

The helical spring should have sufficient elasticity strength to allow the valve seat to open and close at the correct set pressure. The adjustment bolts are located on the top of the body. The valve’s lifting pressure can be adjusted by rotating the screw. The adjusting screw and spring are typically made of steel alloy.

Choosing A Relief Valve

A number of parameters must be considered when selecting a relief valve.

Required Relief Pressures

The anticipated relief pressure is an important consideration, which will be determined by the type of system/machinery in which the relief valve will be installed.

Fluid Property

Prior to deciding on the best materials for your application, you should consider the fluid’s chemical properties. Each fluid has distinct properties and may react differently with various metals and materials. As a result, care must be taken when choosing the body and seal materials that will come into contact with the fluid.

The “wetted” components are the parts of the pressure relief valve that will come into contact with the fluid. For a flammable or hazardous fluid, it is critical to select the proper valve material and design appropriate piping.


As previously stated, fluid property and application will determine the material used for pressure relief valves. Brass, aluminium, and various grades of stainless steel are common pressure relief valve component materials.

Carbon steel or stainless steel springs are commonly used inside relief valves as the valve’s driving component.

Brass is also widely used in common applications and is less expensive. When weight is an issue, aluminium can be used depending on the type of fluid the pressure relief valve will handle. Stainless steel grades are a popular choice for hazardous and corrosive fluids. They also perform well at high operating temperatures.

The seal material’s compatibility with the fluid, as well as the operating temperature range, are also critical.

Flow Rate

The flow requirements, known as flow rate, determine how much pressure must be released from the system once the relief valve is lifted. Flow rate requirements are used to design piping configurations, porting configurations, and effective orifices.

Size and Weight

Many applications have space and weight constraints. For example, even high-pressure machinery such as an air compressor will necessitate a small-sized pressure relief valve when compared to other machinery that requires a larger valve with less pressure relief. 

It is also critical to consider the port (thread) sizes, adjustment styles, and mounting options, as these will have an impact on size and weight.


The materials chosen for the pressure relief valve must not only be compatible with the fluid, but also function properly at the expected operating temperature. The primary concern is whether the chosen elastomer will function properly over the expected temperature range.

Furthermore, in extreme applications, the operating temperature may affect flow capacity and/or spring rate.